Strange! Sterile witch ends in it formerly? (9)
I believe the answer is:
erstwhile
'formerly?' is the definition.
(I've seen this before)
'strange sterile witch ends in it' is the wordplay.
'strange' indicates an anagram.
'ends' means to remove the middle letters.
'in it' means one lot of letters goes inside another.
'witch' with its middle taken out is 'wh'.
'sterile' is an anagram of 'erstile'.
'erstile' going around 'wh' is 'ERSTWHILE'.
(Other definitions for erstwhile that I've seen before include "Former - list where (anag)" , "Formerly" , "In the past" , "Quondam" , "Former, previous" .)